Radiofrequency circuits including an antenna receiving radiofrequency signals and powered by a rechargeable battery are known generally from the prior art. Incorporating a battery charger that uses the energy received via the radiofrequency signals to charge the battery, in order to avoid dismantling the batter, is also known.
One example is given, in particular in U.S. Pat. No. 6,249,212, which discloses a radiofrequency circuit, shown in FIG. 7, on which a bridge 111 formed by PN junction diodes is mounted, allowing the energy received from radiofrequency signals transmitted by a remote reader to be recovered.
The capacitor 117 is used for filtering the harmonics for recovering the continuous energy component. Resistor 121 discharges capacitor 117 in the absence of any electromagnetic field. The time constant created by capacitor 122 and resistor 123 provides a signal to the circuit to inform the latter of the presence of the field.
Diode 115 stores energy in battery 113, which powers the rest of the circuit and prevents the battery being emptied in the absence of any field, i.e. when the level of voltage brought to the battery is lower than the battery charge level. Thus, in this radiofrequency circuit, the battery charge function is achieved by diode 115. The battery charger is thus limited to a single diode.
This battery charger has numerous drawbacks. The pn junction diode component whose two connectors can be connected freely is not available or is poorly characterized in integrated technology. There is thus a fundamental problem of component availability or characterization.
Moreover, the battery charge voltage has to be comprised within relatively specific limits, thus the charge voltage provided by a diode depends upon the voltage applied across the anode thereof, the cathode being connected here to the battery. Consequently, with voltages whose amplitude varies greatly as is the case in our application, i.e. able to exceed 8 volts, it is impossible to use a large capacitance battery as would be desirable. This type of single diode mounting could, at best, be used for recharging a super capacitor, an element that has a much smaller storage capacity, but that tolerates a much greater charge voltage variation. Another alternative would be to calibrate the distance of use of the circuit to prevent any over-voltage, and thus overload, but this is typically the type of constraint that on does not wish to impose on the user. Thus, the use of this type of one diode battery charger leads directly to a problem of battery life and reliability.
Moreover, the diode also has the drawback of not allowing a calibrated current load. The current in a diode varies exponentially as a function of the voltage at the terminals thereof and, thus, exhibits great variations. As it cannot be calibrated, the current also causes a problem of battery life and reliability in the case considered here.
This diode mounting has yet another drawback, linked to the fact that the current-voltage features and, consequently the final battery charge voltage varies greatly with the temperature. The current doubles approximately every 10° C. for a silicon junction, which cannot be tolerated with current proper operating constraints within broad temperature ranges.
Another requirement to be met is having a signal that indicates when the battery charge has finished. In fact, when only one diode is used as the battery charger, this end of battery charge indication can no longer be physically provided by the diode. Moreover, the moment when the diode is no longer charging the battery is not clearly defined. Indeed, the current-voltage feature is exponential, which means that the charging current decreases gradually as the battery is charged, but this does not mean that it is cancelled out. Thus, depending upon the charging current level for a given voltage at the diode anode, there will be a large variation in the voltage at the cathode. For example, for a current changing from 15 μA to 500 nA, a battery voltage variation of the order of 400 mV can be obtained, which cannot be tolerated for most applications.
Wireless battery chargers for recharging batteries that do not require conventional wire recharging are known from the prior art, in particular from US Patent No. 2005/0194926 A1 and GB Patent No. 2 372 534. For this purpose, the batteries are fitted, by means of a contactless battery charger using the energy received from a radiofrequency carrier signal delivered by a remote transmitter. However, no concrete indication is given as to the practical making of such a wireless battery charger, in particular as to the possibility of integrating the latter directly in a monolithic integrated circuit.
Thus, none of the solutions of the prior art presented above is applicable, either because of the numerous drawbacks that they generate, or because of the absence of any concrete solution for the efficient implementation thereof.
One of the problems that arises is thus the making of a contactless battery charger incorporated in a monolithic radiofrequency integrated circuit, using the energy received by an antenna, drawn from radiofrequency signals transmitted by a remote transmitter for recharging the circuit battery. The components outside the monolithic integrated circuit are limited to the antennae, uncoupling capacitors for the various internal supply voltages and the rechargeable battery. The battery charger thus has to be devised fin the same technology as the circuit in which it is integrated, and must not use any external components in order that a reduced number of components is used in the final application.
More specifically, the problem consists in extending the life of this type of battery while optimising the charge, including the charging time. Within the scope of the present invention, two effects likely to substantially reduce the life of a battery have come to light. The first point concerns over-charging the battery, i.e. charging periods when the battery is already completely charged. The second point concerns the charging current applied to the battery terminals during the recharging periods. This current must not depend upon the battery charge level so as to ensure a suitable charging time. Moreover, the current must not have current peaks liable to seriously damage the battery. Indeed, the life of a battery depends not only upon the end of charge voltage value, since any over-charging is detrimental, but also upon the charging current value used for recharging. A current that is too high reduces the life of the battery. This risk arises in particular in the case of a discharged battery to which there is applied a charging voltage equal to the nominal voltage thereof without any current limiting elements. This charging current can then take excessive values. Conversely, the fact of adding a resistive type maximum current limiting element, i.e. a charging current proportional to the voltage at the terminals of the element, leads to a charging current that decreases during the whole of the battery charge period, which extends the battery charge period proportionally, which is evidently also undesirable.